Patent GB-560260-A (GB)
Title: Improvements in or relating to electrical strain gauges
IPC: G01L1/22
Filed: 1942-10-08
Abstract: 560,260. Winding wire and ribbon resistances. AUGHTIE, F., THURSTON, F. R., and COLEMAN, E. P. Oct. 8, 1942, No. 14162. [Class 120 (ii)] [Also in Group XXXVI] A strain gauge comprises a resistance wire or strip wound around and firmly secured to a thin layer of insulating material serving as a support. A fine resistance wire 2, Fig. 1, is wound around a support 1 of thin paper or fabric and is protected by cover layers 4, 5 of thin paper, the whole being bonded together with insulating material such as a phenol formaldehyde varnish. End connections are made by relatively stout leads 3 of ribbon conductor. The resistance gauge is attached to a specimen or object by means of an adhesive such as cellulose acetate cement or fish glue so as to be strained with it, the strain being measured by determining the change in resistance. If the gauge is to be used at a high temperature the support may be of asbestos paper and the bonding agent a stoving or firing glaze. The resistance may be wound in a winding machine comprising a frame 6, Fig. 2, supporting a spindle 7 and a lead screw 8 coupled together by a belt 11 passing over pulleys 9, 10. The lead screw 8 carries a nut 13 provided with a guide 14 for the resistance wire, while a spindle 15 supports a reel 16 from which the resistance wire is wound. A tail spindle 17 is provided coaxial with the spindle 7 and flats 18, 19 are cut on the spindles 7, 17 to which the support is secured by screws and nuts 20, 21. A thin steel strip 22 may be provided for stiffening the support during winding, in which case the support 1 is formed of a loop of paper which may be slipped over the strip 22 or produced by wrapping the paper round the strip with a narrow overlap 23. The strip 22 may be slightly tapered to facilitate removing the support 1 after winding. Alternatively the winding may be carried out with the support in cylindrical form, flattening being subsequently effected. A cylindrical mandrel 24, Fig. 3, formed with a narrow slot 25 may be employed. The tail spindle has an end section 26 which is inserted into the mandrel for winding, its diameter being such as to expand the mandrel slightly. After winding and removal from the mandrel the wound support may be flattened between plates or by passing between rollers, or may be passed over a jig, Fig. 4, having a cylindrical portion 27 which will easily enter the support, followed by an intermediate section 28 of constant perimeter which leads smoothly to a flat portion 29 wide enough to hold the wound support flat. The support then passes to rollers which complete the flattening operation. The leads 3 may be secured by soldering or spot welding. In order to localise the bond between the leads and the support or cover layers, a thread of cotton may be laid across the leads immediately outside and at right angles to the gauge length. Multiple gauges may be formed with two or more resistances extending in different directions, for example at right angles. If the supports are of single thickness a layer of paper is interposed between the two windings as well as cover layers on the outer faces, the whole being bonded together. If each gauge is wound on a loop of paper, one loop is slightly opened and the other slipped through it. The gauge may be shaped during the process of bonding into a form suitable for attachment to the test surface when this is not flat or of simple and restricted curvature. The test surface itself may form one part of a mould and the process can then be adapted so that the gauge is secured to the test surface during the period of bonding together the composite parts of the gauge.